1. Field of the Invention
The present invention relates generally to high temperature polymers. It relates particularly to a PMR polyimide approach to the obtainment of tough, low toxicity, easy-to-process high temperature polymers.
2. Description of the Related Art
In-situ polymerization of monomer reactants (PMR) type polyimides constitute an important class of ultra high performance composite matrix resins. Their graphite fiber reinforced composite materials are finding increased use in various aircraft engine components, which operate at temperatures ranging from 232.degree. C. to 371.degree. C. for several hundred to several thousand hours. PMR-15 (15 stands for a formulated molecular weight of 1500 between crosslinks) is the best known and most widely used PMR polyimide. Its attributes include relatively easy processing, substantially lower costs, and excellent property retention at elevated temperatures, compared to other commercially available high temperature materials. This material is prepared from an alcohol solution of three monomers: monomethyl ester of 5-norbornene-2,3-dicarboxylic acid (NE), 4,4'-methylenedianiline (MDA), and dimethyl ester of 3,3', 4,4'-benzophenonetetracarboxylic acid (BTDE) as follows: ##STR1## The monomer solution is impregnated into a reinforcing fiber, and in-situ polymerization of the monomer reactants occurs directly on the fiber surface to afford a composite material having good thermal and mechanical performance.
However, in recent years the health and safety problems of MDA and polymers derived from this diamine have become a great concern. On the basis of extensive testing data, NIOSH recommends that MDA be considered as a potential occupational carcinogen and that occupational exposure be minimized. (Current Intelligence Bulletin 47, NIOSA, Jul. 25, 1986). Increasing environmental restriction has, therefore, made the MDA containing PMR-15 less acceptable.
Attempts have been made to replace the MDA in PMR-15 with a less toxic diamine. However, when a diamine containing a non-benzylic linking group, such as ether or carbonyl, was used to replace MDA, the MDA-replaced material invariably exhibited significantly greater 316.degree. C. resin weight loss, compared to MDA-based PMR-15, thus making the new material much less effective. ([W. B. Alston, Polymer Preprints, 27 (2) pp. 410-411 (1986); NASA TM-100791 AVSCOM TR-88-C-004, 1988; and Proceedings of the 18th International SAMPE Technical Conference, 18, pp. 1006-1014 (1986)] and P. Delvigs, D. L. Klupotek and P. J. Cavano, Proceedings of the Symposium on Polyimides and other high performance polymers sponsored by the ACS Polymer Chemistry Division, Jan. 22-25, 1990 in San Diego, CA). Other work has also been done in developing MDA-replaced PMR materials that show composite elevated temperature mechanical properties either comparable or significantly lower than PMR-15, particularly at 316.degree. C. or higher temperatures. These new materials include British Petroleum's B1 formulation (N. D. Hoyle, et. al., Proceedings of the 22nd International SAMPE Technical Conference, 22 pp. 198-212 (1990)), Ferro's CPI-2310 (R. B. Baggett, H. K. Gupta and M. R. Kantz, Proceedings of the 35th International SAMPE Technical Conference, 35, pp. 1555-1565 (1990)), and United Technologies' 3,3'-DDS-PMR-16.5 (D. A. Scola, Proceedings of the High Temple Workshop, Cocoa Beach, FL, Jan. 29-Feb. 1, 1990).
Unlike MDA, 3,4'-oxydianiline (3,4'-ODA) has been found to exhibit no detectable mutagenicity according to the Ames TA 100 test with salmonella typhimurium. (W. D. Ross, et. al., NASA Contract Report 166085 (1983).) However, when the TA 98 method was used, moderate mutagenicity was detected for both 3,4'-ODA and MDA. From these tests, the conclusion can be drawn that 3,4'-ODA is at least potentially less toxic than MDA, and that a polymer made from 3,4'-ODA is potentially less toxic than one made with MDA. However, 3,4'-ODA has not been successfully used to make a useful PMR polyimide prior to the instant undertaking.
Powder technology has also been used in an attempt to eliminate the toxicity associated with MDA. In this case, the three monomer reactants of PMR-15 are converted into a fully imidized PMR-15 powder, which theoretically contains no free MDA. The powder is then used to prepare solventless prepreg, which is subsequently consolidated into a composite, using standard compression or autoclave molding techniques. (T. Hartness, D. Porter and J. P. Reardon, Proceedings of the 34th SAMPE Symposium, 34, pp. 112-126 (1989).) The composite fabricated from fully imidized PMR-15 showed mechanical properties that compared favorably with conventional solution processed counterparts, but with much less ease of processing.
One other approach has been used to develop an MDA-free PMR system. A new proprietary dimer is used to develop what is now called PMR-15-MDAF (MDA-free). (F. Riel, T. Vuong and E. Delaney, Patent Pending, Rohr Industries, appeared in Performance Materials, Oct. 1, 1990). The properties of this new material remain undisclosed, however.
None of these prior art products have disclosed the desired combination of properties set forth hereinabove, including greater ease of processing, such as a broader processing window, better reproducibility of high quality composite parts, better elevated temperature mechanical properties, and higher retention of mechanical properties at an elevated temperature, particularly, at 371.degree. C.